1. Field
This disclosure is related to printed circuit boards (PCBs). More specifically, this disclosure relates to a printed circuit board which is configured to minimize skew between parallel signal traces.
2. Related Art
Advances in fabrication technologies continue to increase the performance and maximum throughput of circuit devices. However, enabling the circuit devices to maintain high throughput generally requires that circuit devices communicate with one another at high data rates. The requirement for high data-rate communication between circuit devices is increasing demand for printed circuit boards (PCBs) with interconnects (i.e., signal wires/traces fabricated in the PCB) that can carry communication signals between the circuit devices at high data rates.
One common problem with communicating signals at high data rates in PCBs is skew between the signals. More specifically, many PCBs include pairs of interconnections that simultaneously propagate corresponding pairs of signal values (e.g., a clock/data pair of signals). However, because the propagation time of signals traveling in the interconnect pair can be different due to physical differences in each signal trace in the interconnect pair, the signals can experience different propagation times (i.e., propagation skew), which can cause the circuit devices that depend on the signals to operate incorrectly.
Because many PCBs include two or more layers in which signal traces are routed, one of the main sources for propagation skew is electromagnetic interference between signals on neighboring trace layers of a PCB. More specifically, due to the physical properties of signal traces in these layers (e.g., capacitance and inductance), signals on one trace layer of the PCB can be affected by signal traces in a neighboring trace layer of the PCB. To minimize this cross-layer interference, some PCBs include a laminate layer with an embedded fiberglass weave that has a high dielectric constant. The embedded fiberglass weave helps to reduce the capacitive coupling of signals between the layers.
Unfortunately, even with the embedded fiberglass weave, skew can still be a problem in PCBs. This problem still arises because the dielectric constant can vary across a trace layer of a PCB when some points of the trace layer are near a resin-rich portion of the PCB, while other points are near a fiberglass-rich portion of the PCB. Even these small variations in the laminate layer's dielectric constant can cause sufficient skew between signals to cause errors in high-performance circuit devices.